The present invention relates to an improvement in a catalyst for treating exhaust gas discharged from an internal combustion engine to remove pollutants from the exhaust gas. More particularly, the present invention relates to an improvement in a process for preparing a thermally stable catalyst used in the treatment of vehicle exhaust gas. As well known, there is disclosed a catalyst including one or more kinds of precious metals dispersed within the internal space of a high surface area support and on the surface thereof. A refractory ceramic material-made monolithic carrier is coated with the precious metal-dispersed support in the form of slurry (washcoat). As the support, an activated alumina support, particularly, a high surface area alumina support having a BET surface area of 60 m2/g or more, for example, 150 m2/g or more or 200 m2/g or more, is used. Here, activated alumina is a mixture of gamma-alumina and delta-alumina. A typical catalyst is composed of a small amount of a platinum-group metal such as platinum or palladium dispersed in the internal space of an activated alumina support and on the surface thereof, and additionally includes at least one of rhodium, ruthenium and iridium. An alumina support containing such a catalytically active component is applied onto a honeycombed monolithic carrier provided with many gas channels in the form of fine slurry, called washcoat. The common problem with catalysts for exhaust gas treatment is that an activated alumina support is thermally decomposed when a catalyst is exposed to high-temperature exhaust gas of an internal combustion engine for a long period of time. For example, the temperature of automobile exhaust gas may be 1000° C. or more. Due to the high temperature, an activated alumina support is constricted in volume under the presence of vapor and is converted into a low surface area alumina support, and thus catalytically active components are confined in the constricted support, so the exposed surface area of a catalyst is reduced, thereby deteriorating catalytic activity. Further, owing to the high-temperature exhaust gas, catalytically active components may be separated from the alumina support or may be transposed. Therefore, in a process of preparing a catalyst, it is required that catalytically active components be strongly fixed on the alumina support.
There are disclosed methods of preparing an alumina support and processes of fixing catalytically active components onto a support. For example, U.S. Pat. No. 6,288,007 discloses an alumina support having a surface treated with silica by impregnation or vapor deposition. In the impregnation, first, a silicon compound solution is formed, an alumina support makes contact with the solution, and the support is treated with air or water, thus converting silicon compound into silicon oxide (silica). Further, Korean Patent Application Publication No. 2011-0109625 discloses a method of preparing mesoporous alumina, including the steps of: mixing a surfactant and an alumina precursor with alcohol and distilled water, stirring the mixture and then separating solid matter; and drying and calcinating the solid matter.